In the actuator control of a magnetic recording apparatus, particularly a hard disk drive (HDD), aliasing caused by mechanical resonances of a head suspension would be a problem. To solve the problem, an analog notch filter turned to a resonance point is widely used. This would be effective if the resonance frequency does not vary. However, the resonance frequency may vary to a certain degree to deviate from the characteristic of the notch filter. Therefore, in the method using the conventional notch filter, it is necessary to strictly control resonance modes of the head suspension, which involves a problem of cost and quality.
IBM Technical Disclosure Bulletin, Vol. 33, No. 3A, August 1990, pp. 222-223, proposes that an elliptic filter may be used instead of the conventional notch filter. The elliptic filter has an advantage that it introduces the least amount of phase-lag which, however, cannot be zero. The elliptic filter involves a problem also if resonance frequencies vary widely and is expensive as compared with the conventional notch filter. The above literature describes that the use of low-pass filters is generally ruled out since they cause very large phase shifts at the servo zero db crossover frequency (ZCF).
In respect of digital control, for example, U.S. Pat. No. 4,398,228 proposes to remove the effect of mechanical resonances by employing the same sampling and mechanical resonance frequencies. Since the sampling frequency is uniquely determined based on the rotational speed of and the number of sectors on a disk, the resonance frequency is practically adjusted to the sampling frequency. However, it is difficult to keep the resonance frequency constant.